Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by the narrowing or blockage of an artery leading to heart tissue. If sufficiently severe, cardiac ischemia results in an acute myocardial infarction (AMI), also referred to as a heart attack. With AMI, a substantial portion of heart muscle ceases to function because it no longer receives oxygen, usually due to complete or non-complete blockage of the coronary artery. Generally, AMI occurs when plaque (such as fat, cholesterol, and calcium) builds up and then ruptures in the coronary artery, allowing a blood clot or thrombus to form. Eventually, the blood clot completely blocks the coronary artery and so heart tissue beyond the blockage no longer receives oxygen and the tissue dies. In many cases, an AMI proves fatal because too much tissue is damaged to allow continued functioning of the heart muscle. Indeed, AMI is a leading cause of death here in the United States and worldwide. In some cases, although the AMI itself is not fatal, it strikes while the victim is engaged in potentially dangerous activities, such as driving vehicles or flying airplanes, and the severe pain and possible loss of consciousness associated with AMI results in fatal accidents. Even if the victim survives the AMI, quality of life may thereafter be severely restricted. Often AMI is preceded by episodes of cardiac ischemia that are not sufficiently serious to cause actual permanent injury to the heart tissue. Nevertheless, these episodes are often precursors to AMI. Episodes of cardiac ischemia may also trigger certain types of arrhythmias that may prove fatal, particularly ventricular fibrillation (VF) where the ventricles of the heart beat chaotically, resulting in little or no net flow of blood from the heart to the brain and other organs.
Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein, or are candidates for such devices. Accordingly, techniques have been developed for using the implanted medical device to generate diagnostic information pertaining to ischemia for subsequent review by a physician or other clinician. That is, the diagnostic information is transmitted from the implanted device to a bedside monitor, external programmer or other external device, which displays the diagnostic information to the physician. The physician then reviews the information to determine whether the ischemia is “clinically actionable”, i.e. whether medical intervention is warranted. For example, the diagnostics might specify an “ischemic burden”, which is representative of a change in duration and/or severity of ischemic events over time as determined, e.g., based on changes in ST segment elevations within an intracardiac electrogram (IEGM). If the ischemic burden increases significantly within a particular patient, medical intervention might be necessary.
However, various other factors are preferably considered by the physician along with duration and severity of episodes of ischemia when determining whether the ischemia is clinically actionable. Accordingly, it would be desirable to provide diagnostic systems capable of generating enhanced ischemia diagnostics that additionally provide information pertaining to these other factors. It is to this end that aspects of the present invention are directed.
Some implantable systems are programmed to directly detect cardiac ischemia based on ST segments and to immediately generate ischemia warning signals for alerting the patient. That is, the implanted device does not merely generate ischemia diagnostics for subsequent review, but actually detects the onset of episodes of ischemia so that warnings can be promptly generated. Some such systems automatically control or adjust therapy in response to the ischemia as well. See, for example, U.S. Pat. No. 6,108,577 to Benser, entitled “Method and Apparatus for Detecting Changes in Electrocardiogram Signals”. See, also, U.S. Pat. Nos. 5,113,869 to Nappholz; 5,135,004 to Adams et al.; 5,199,428 to Obel et al.; 5,203,326 to Collins; 5,313,953 to Yomtov et al; 6,501,983 to Natarajan, et al.; 6,016,443, 6,233,486, 6,256,538, and 6,264,606 to Ekwall; 6,021,350 to Mathson; 6,112,116 and 6,272,379 to Fischell et al; 6,128,526, 6,115,628 and 6,381,493 to Stadler et al.
As noted, however, there are various other factors, discussed below, that are preferably considered before determining whether cardiac ischemia is clinically actionable. Accordingly, those factors ought to be taken into account by the physician in the management of the patient. Hence, it would be desirable to provide implantable devices with the capability of generating diagnostics for physician review that take the additional factors into account. It is to this end that aspects of the present invention are primarily directed. Also, if the implantable device itself is equipped to generate warnings and control therapy in response to ischemia, such factors ought to be taken into account by the implanted device as well. Accordingly, it would also be desirable to provide implantable devices that are capable of considering these factors before generating warnings or delivering therapy. It is to this end that still other aspects of the present invention are directed.